Current-injection spark source for emission spectroscopy

ABSTRACT

An analytical spark gap is provided between the conductors of a resonant line at one end thereof. The other end of the line is connected to a radio frequency power source which can be electronically pulsed. The conductors of the line are preferably in the form of inner and outer coaxial cylinders. The spark gap is preferably between a first electrode at one end of the inner conductor and a second axial electrode connected by means of an end wall to the outer electrode. A current injection input lead is connected to an intermediate point along the line, so as to provide an effective length of one-quarter wavelength between the injection point and the spark gap. In this way, a node is produced at the injection point. Around the spark gap, the outer conductor forms a cylindrical chamber, in which acoustical resonances are set up, with resulting stabilization of the spark in that the spark is held stationary in an axial position. Optical resonance can also be produced within the chamber by providing a cylindrical mirror on the inside of the outer cylindrical conductor around the spark gap.

United States Patent Walters et al.

[ 51 Apr. 4, 1972 [54] CURRENT-INJECTION SPARK SOURCE FOR EMISSIONSPECTROSCOPY [72] Inventors: John P. Walters, Madison, Wis.; Thomas V.Bruhns, Lacey, Wash.

Wisconsin Alumni Research Foundation, Madison, Wis.

22 Filed: Feb. 4, 1970 21 Appl.No.: 8,462

[73] Assignee:

[52] U.S. Cl. ..356/86, 313/231, 313/325,

2,929,953 3/1960 Mitteldorf et al. ...356/86 UX 3,513,516 5/1970 Oddo etal. ..3l3/325 X OTHER PUBLICATIONS Walters, Analytical Chemistry, Vol.40, No. 11, September 1968, pages 1672- 1681 Primary Examiner-Ronald L.Wibert Assistant Examiner-F. L. Evans AttorneyBurmeister, Palmatier &Hamby [5 7] ABSTRACT An analytical spark gap is provided between theconductors of a resonant line at one end thereof. The other end of theline is connected to a radio frequency power source which can beelectronically pulsed. The conductors of the line are preferably in theform of inner and outer coaxial cylinders. The spark gap is preferablybetween a first electrode at one end of the inner conductor and a secondaxial electrode connected by means of an end wall to the outerelectrode. A current injection input lead is connected to anintermediate point along the line, so as to provide an effective lengthof onequarter wavelength between the injection point and the spark gap.In this way, a node is produced at the injection point. Around the sparkgap, the outer conductor forms a cylindrical chamber, in whichacoustical resonances are set up, with resulting stabilization of thespark in that the spark is held stationary in an axial position. Opticalresonance can also be produced within the chamber by providing acylindrical mirror on the inside of the outer cylindrical conductoraround the spark gap.

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CURRENT-INJECTION SPARK SOURCE FOR EMISSION SPECTROSCOPY The inventiondescribed herein was made in the course of or under a grant from theNational Science Foundation, an agency of the United States Government.

This invention relates to the production of spark discharges foranalytical and other purposes in connection with optical emissionspectroscopy.

One object of the present invention is to provide new and improvedapparatus whereby spark discharges are initiated by high voltages atradio frequencies, but the major spark currents are injected into thespark gap from an auxiliary source.

A further object is to provide new and improved apparatus whereby theradio frequency source and the injection current source are effectivelyisolated from each other, despite the connection of both to the sparkgap.

Another object is to provide such a new and improved apparatus in whichthe radio frequency source can be pulsed electronically and at a lowerpower level.

It is a further object to provide such a new and improved apparatus inwhich the high radio frequency voltage is produced by a radio frequencysource working into a resonant output line.

Another object is to providenew and improved apparatus whereby anatmospheric spark discharge is stabilized so that it is held stationaryon the spark gap electrodes.

Accordingly, the present invention preferably comprisesan axial sparkgap in a coaxial configuration, whereby the spark gap is disposedaxially in a cylindrical chamber. The spark is stabilized. and heldstationary by acoustical resonances in the chamber, due to acousticalreflections from the cylindrical wall of the chamber.

The spark is preferably initiated by a radio frequency source, connectedto one end of a resonant line, the spark gap being connected to theopposite end. The line conductors are preferably in the form of coaxialcylinders. One of the axial spark gap electrodes is connected to theinner conductor of the line, while the other electrode is preferablyconnected to an end well, and then to the outer cylindrical conductor.

A current injection lead is connected to the axial line conductor at anintermediate point along the line. Preferably, the electrical length ofthe line, between the injection point and the spark gap, issubstantially one-fourth wavelength. In this way, a voltage node orminimum is produced at the injection point, so that the injectioncurrent source is effectively isolated from the radio frequency source.

The radio frequency source is arranged so that it can be pulsedelectronically, with pulses of low power. In this way, sparks canreadily be produced with a high repetition rate. Because of the lowpower requirements, the triggering pulses can be derived directly from acomputer.

The present invention has the advantage of making it possible to timethe breakdown of the spark gap with a high degree of precision. Suchprecise timing makes it easy to synchronize the spark with other deviceswhich must be properly phased with respect to the spark. Moreover, theaccurate spark ignition, possible with the spark source of the presentinvention, may be useful in studying the mechanism by which a spark gapis broken down.

The present invention makes it easy to deliver a wide variety of currentpulses to the formed spark. The current injection system affords lowimpedance to current delivery. Moreover, the voltage level at theinjection point is low. A wide variety of current sources can beconnected to the injection point. Low voltage components can be employedin the current injection devices. The inductance between the injectioncurrent source and the spark gap is low. It is not necessary to use highinductance filters between the injection current source and the sparkgap.

The cylindrical outer conductor of the coaxial line provides completeelectrical shielding around the spark gap. Accordingly, the spark gap isnot appreciably affected by external electric or magnetic fields.Moreover, the radiation of radio frequency energy from the spark gap isminimized.

The cylindrical outer conductor of the resonant line produces acousticalresonances around the spark gap, which has the effect of stabilizing thespark so that it stands still, particularly when the spark repetitionrate is fairly high. The stabilization of the spark makes it possible toachieve optical resonance in the cylindrical chamber around the sparkgap. In this way, it becomes possible to generate laser beams emanatingfrom the spark.

The system operates at atmospheric pressure, yet produces a stable,unwavering spark across the spark gap. The acoustical resonance, wherebythe spark is stabilized, involves the action of the shock waves from thespark. If desired, the system can be employed as a source of such shockwaves.

The system of the present invention is especially useful for makingelectrochemical analyses, in which the materials to be analyzed arevaporized by the action of the spark. The light emitted by the spark canbe used for optical emission spectroscopy. In addition, the spark can beused as a vapor or ion source in connection with mass spectroscopy.

Solid materials to be analyzed can be placed on the spark gapelectrodes. Gases to be analyzed can be introduced into the sparkthrough one or more openings in one of the electrodes.

The system of the present invention is also useful for the controllederosion of various materials by the action of the spark. The sparkrepetition rate can be high, yet extremely high currents can be injectedfrom a low voltage source.

Further objects, advantages and features of the present invention willappear from the following description, taken with the accompanyingdrawings, in which:

FIG. 1 is a block diagram of a spark source to be described as anillustrative embodiment of the present invention.

FIG. 2 is a longitudinal section of the quarter-wave line and associatedcomponents, employed in the spark source of FIG. 1.

FIG. 3 is an enlarged elevational section, showing the spark gapelectrodes and the associated adjusting mechanism.

FIG. 4 is a fragmentary enlarged section of one of the spark gapelectrodes.

FIGS. 5a and 5b are approximate equivalent circuit diagrams of thequarter-wave line and associated components.

FIG. 6 is a circuit diagram of the output stage of the radio frequencysource.

FIG. 7 is a circuit diagram of the pulsed driver stage of the radiofrequency source.

FIG. 8 is an oscillogram of the radio frequency voltage as the spark gapis broken down.

FIG. 9 is an oscillogram of the injection current across the spark gapwith a fully oscillatory discharge.

FIG. 10 is an oscillogram representing an enlargement of the initialportion of the discharge shown in FIG. 9.

FIG. 11 is an oscillogram of the injection current due to aunidirectional discharge pulse.

FIG. 12 is an oscillogram of the injection current for a half sinusoidaldischarge current.

FIG. 13 is another oscillogram of discharge current representing aunidirectional pulse from a low impedance line with distributedparameters.

FIG. 14 is an oscillogram representing the parallel combination of thesystems which produced the pulses of FIGS. 12 and 13.

FIG. 15 is an oscillogram of a low current, non-reversing pulse.

FIG. 16 is an oscillogram of a medium current, reversing pulse.

FIG. 17 is an oscillogram representing an enlargement of the leadingportion of the reversing pulse of FIG. 16.

It will be seen that FIGS. 1 and 2 illustrate a system 20 for producingrepetitive sparks. Thus, a spark gap 22 is provided between upper andlower electrodes 24 and 26. In this case, the upper electrode 24 is inthe fonn of a rod with a flat lower end 28. However, the upper electrodemay assume a variety of other forms, such as a flat disc, for example.The lower electrode 26 is illustrated as being sharply pointed but hereagain the form of electrode can be varied.

Either or both of the electrodes can be made of a material to beanalyzed. Alternatively, a quantity of such material can be placed onone or both of the electrodes.

FIG. 4 illustrates means for introducing gases into the spark. It willbe seen that the lower electrode 26 is formed with an axial passage 30.The electrode 26 is cylindrical in form with a tapered upper end portion31. In this case, the electrode 26 is mounted for sliding adjustmentwithin a sleeve 32. The gas to be analyzed is introduced into the sleeve32 through a side tube 34. The gas passes upwardly through the axialopening 30 and around the outside of a needle or pin 36 mounted in theopening 30. The pin 36 is smaller in diameter than the opening 30. Asshown, the pin 36 is secured within the opening by means of a pluralityof set screws 38. The pin 36 has a sharp point 40 at its upper end tofacilitate the ignition of the spark.

In order to provide for ignition of the spark by radio frequencyvoltages, the spark gap 22 is effectively connected between theconductors of a radio frequency transmission line 42. The spark ignitionvoltage is built up by producing standing waves along the line. Whilethe line.42 may be of various forms, it is shown as being of the coaxialtype, having inner and outer cylindrical conductors 44 and 46. It willbe seen that the inner conductor 44 is in the form of a rod or tube ofrelatively small diameter, while the outer conductor 46 cpmprises a tubeor cylinder of large diameter. The inner conductor 44 is made as smallas possible, consistent with the ability to carry substantial currents,so that the characteristic impedance of the line will be high. In thisway, less energy will be required to build up standing waves ofsufficient voltage to break down the spark gap. To provide foradjustment of the line 42, the inner conductor 44 may comprisetelescopically joined sections 44a and b. In the illustratedconstruction, the upper section 44a is slidably received within thelower end of the sleeve 32. Support for the sleeve 32 is provided by adisc or partition 48, preferably made of an insulating material with avery low loss factor. The partition 48 is mounted within the outercylindrical conductor 46. The sleeve 32 is slidable through an axialopening 50 in the partition 48.

It will be understood that the line conductors 44 and 46 are made ofcopper or other highly conductive materials. Support for the upperelectrode 24 is provided by an end wall or disc 52, closing the upperend of the outer cylindrical electrode 46. The end wall 52 is also madeof a highly conductive material and is preferablymade so that it can beremoved from the outer cylinder 46. The electroderod 24 is connectedelectrically to the end wall 52, which in turn is connected to the outercylindrical conductor 46.

To provide for precise adjustment of the electrode 24, it is connectedto a micrometer screw 54 having its counterpart threaded element 56mounted on the end wall 52 by means of a mounting plate 58. Theelectrode 24 can be advanced or retracted by rotating the screw 54 inopposite directions.

Additional support for the lower member 44b of the axial conductor 44 ispreferably provided by another partition 60, similar to the partition48. The partition 60 is made of insulating material and is mountedwithin the outer cylindrical conductor 46. The member 44b extendsthrough an axial opening 62 in the partition 60.

It will be recalled that the spark gap 22 is connected to one end of theresonant transmission line 42. The other end of the line is connected toa radio frequency source 64, indicated diagrammatically in FIG. 2. Thesource 64 may include an output anode terminal 66, adapted to supply aradio frequency voltage to the axial conductor 44. It is preferred toconnect one or more blocking capacitors 68 between the anode terminaland the axial conductor 44 so that the direct voltage on the anodeterminal 66 will not be applied to the axial conductor 44. Three suchcapacitors 68, connected in parallel, are shown in FIG. 2. Thecapacitors 68 are connected between the anode terminal 66 and a disc orplate 70, welded or otherwise secured to the lower end of the axialconductor 44.

A current injection lead 72 is connected to an intermediate point alongthe inner conductor 44 of the coaxial transmission line 42. The lead 72extends radially out of the outer cylindrical conductor 46 through aninsulator 74.

The effective length of the line between the spark gap 22 and theinjection lead 72 is one-fourth of a wavelength. Due to the reflectionof traveling waves along the line from the spark gap, a standing waveexists along the transmission line 42, with a node at the injectionpoint, where the injection lead 72 is connected to the axial conductor44. Thus, the radio frequency voltage is at a minimum and is very low atthe injection point. Accordingly, the injection lead 72 is effectivelyisolated from the radio frequency source 64.

The length of the coaxial transmission line 42 between the anodeterminal 66 and the spark gap 22 is illustrated as somewhat greater thanone-quarter wavelength. Thus, the line 42 presents an inductiveimpedance at the anode terminal 66.

One or more tuning devices are preferably provided to tune the coaxialline 42 to a resonant condition. As illustrated, this tuning adjustmentis provided by a variable capacitor 76, connected between the inner andouter conductors 44 and 46, near the radio frequency source 64. In thiscase, the variable capacitor comprises a stationary plate 78, connectedto the axial conductor 44, and a movable plate-80, mounted on the outercylindrical conductor 46. Support for the plate is provided by anadjusting screw 82, threaded through a sleeve 84 on the outercylindrical conductor 46. The screw 82 may be provided with a manuallyrotatable adjusting knob 86.

The telescopically adjustable sections 440 and b of the axial conductor44 make is possible to adjust the effective length of the coaxialtransmission line 42. This adjustment affects the location of the node,and. also the resonant input impedance of the line 42. I

Additional details of the apparatus will be evident from FIG. 1, whichis a block diagram of the system. In this case, the radio frequencysource 64 provides an output to the resonant line 42 at 162 MHz.However, the particular frequency is merely a matter of convenience andcan be varied over a wide range. Moreover, the details of the radiofrequency source 64 are subject to wide variations.

In the illustrated system, to be described by way of example, the outputat 162 MHz. is derived by starting with a relatively low frequencysignal and utilizing a series of frequency multipliers. Thus, as shownin FIG. 1, the system comprises a master crystal oscillator whichprovides an output at 6.75 MHz. This signal is amplified by a tunedamplifier 92 operating at 6.75 MHz. The amplifier 92 drives a frequencydoubling amplifier 94 which delivers an output at 13.5 MHz. This outputdrives another frequency doubling amplifier 96, providing an output at27 MHz.

The signal at 27 MHz. drives still another frequency doubling amplifier98 providing an output at 54 MHz. This output is fed to a frequencytripling amplifier 100, developing an output at 162 MHz.

The 162 MHz. signal drives a push-pull gated power amplifier 102 whichis adapted to be pulsed, as will be described in detail presently.The-pulsed output drives the final power amplifier 104 having its outputconnected to the anode terminal 66, shown in FIG. 2. Thus, the poweramplifier 104 supplies its pulsed output to the quarter-wave resonantline 42, which develops a high radio frequency voltage across spark gap22.

The arrangement for pulsing the gated power amplifier 102 is alsosubject to wide variations. As shown, the gated power amplifier 102 iscontrolled by a pulser 106 utilizing an electronic switching element,such as a silicon controlled rectifier, as will be described in detailpresently. The pulser 106 is driven by a trigger generator 108, which isadapted to respond to an external source 110 of triggering signals. Suchexternal source may take the form of a computer, inasmuch as therequired signal level is very low.

As previously indicated, an injection current source 112 is connected tothe resonant line 42 by means of the input lead 72. The source 112 isenergized by a power supply 114. The injection current source 112 mayassume a wide variety of forms. For example, it may comprise a capacitorwhich is f charged by the power supply 114 and is discharged into theresonant line 42 and thence across the spark gap 22. Various elementsmay be connected to the capacitor to change the wave form of thedischarge current.

It will be recalled that the current injection lead 72 is connected tothe resonant line 42 at a point where the radio frequency voltage is ata minimum. Thus, only a minimum of filtering is required to isolate theinjection current source 112 from the radio frequency source 64. Asshown in FIG. 6, such filtering is provided by small inductances 116,118 and 120, together with a small bypass capacitor 122. The inductance116 represents the distributed inductance of the current injection lead72. Each of the inductances 118 and 120 may com-' prise a coil havingone turn or at most a few turns of wire. The capacitor 122 is connectedto ground from the junction 124 between the inductances 118 and 120. Theinjection current from the source 112 may be delivered to a jack orconnector 126, as shown in FIG. 6.

The details of the final power amplifier 104 are shown in FIG. 6, but itwill be understood that such details are subject to wide variation. Asshown, the power amplifier 104 utilizes a beam power tube 130 having itsanode connected to the anode terminal 66, and thence through theblocking capacitors 68 to the axial conductor 44 of the resonant line42. The energizing voltage may be supplied to the anode by a lead 132running along the axial conductor 44 to a point adjacent the currentinjection lead 72. The lead 132 is then connected through smallinductances 134 and 136 to the high voltage anode supply lead 138. Theinductances 134 and 136 have a filtering action which is aided by bypasscapacitors 138 and 140, connected between ground and the opposite endsand the inductance 136.

The pulsed input power at 162 MHz. is supplied to the grid of the tube130 by a transformer 144 having primary and secondary windings 146 and148. A variable tuning capacitor 150 is connected in series with theprimary winding 146. The secondary winding 148 has a tap 152 which isconnected to a bias supply lead 154 through filtering inductances 156and 158. Bypass capacitors 160, 162 and 164 are connected to theopposite ends of the inductance 158.

One end of the secondary winding 148 is connected to the control grid ofthe tube 130. The other end is connected to a neutralizing probe 166extending into the outer cylindrical conductor 46, to a point near theanode terminal 66. A variable neutralizing capacitor 168 is connected toground from the last mentioned end of the secondary winding 148.

The screen grid of the tube 130 is connected to a power supply lead 170through filtering inductances 172 and 174, aided by bypass capacitors176, 178, 180 and 182.

The cathode and one side of the filament of the tube 130 are grounded.The other side of the filament is connected to a power supply lead 184through filtering inductances 186 and 188, aided by bypass capacitors190, 192 and 194.

FIG. 7 shows details of the push-pull gated power amplifier 102 and theSCR pulser 106. It will be understood that the details of these circuitsare subject to wide variations. The illustrated amplifier 102 utilizes adual beam power tube 200 having its control grids connected to thecenter tapped secondary winding 202 of an input transformer 204, whichalso has a center tapped primary winding 206. It will be understood thatinput power at 162 MHz. is supplied to the primary winding 206. Thecenter tap 208 of the secondary winding 202 is connected to groundthrough biasing resistors 210 and 212. A bypass capacitor 214 isconnected across the resistor 212.

The anodes of the dual tube 200 are connected to the center tappedprimary winding 216 of an output transformer 218, which also has asecondary winding 220. It will be understood that the output leads 222and 224 are adapted to be connected to the input of the final poweramplifier 104. A tuning capacitor 226 is connected in series with thesecondary winding 220.

The primary winding 216 is tuned by dual variable capacitors 230,connected between the anodes and ground. The cathodes of the dual tube200 are also grounded.

The anodes and the screen grids of the dual tube 200 are adapted to bepulsed by the pulsing circuit 106. It will be seen that the pulsingcircuit 106 utilizes a silicon controlled rectifier (SCR) 234. Thepositive power supply lead 236 is connected to the anode of the SCR 234through an inductor 238 and a resistor 240. Bypass capacitors 242, 244and 246 are connected between the power supply lead 236 and ground. Adischarge capacitor 248 is connected between the anode of the SCR 234and ground.

The cathode of the SCR 234 is connected to the center tap of the coil216 through a radio frequency choke coil 250. The screen grids of thedual tube 200 are connected to the cathode of the SCR 234 throughparallel resistors 252 and 254, bypassed by a capacitor 256. A radiofrequency bypass capacitor 258 is connected between the screen grids andground.

When the SCR 234 is pulsed into conductivity, the anodes and screengrids of the dual tube 200 are energized momentarily by the discharge ofthe capacitor 248, following which the SCR becomes non-conductive,because the current through the resistor 240 is insufficient to maintainconduction in the SCR. The capacitor 248 is then recharged through theresistor 240.

The control circuit for the SCR 234 comprises a transformer 260 havingprimary and secondary windings 262 and 264. The primary winding 262 isadapted to receive triggering signals from input leads 266 and 268. Thesecondary winding 264 is connected between the gate and the cathode ofthe SCR 234. A damping resistor 270 is connected across the secondarywinding 264.

During normal operation, the SCR 234 is pulsed by the triggering signalsat the input leads 266 and 268. However, when desired, the dual tube 200can be energized continuously. This is accomplished by means of a switch274 comprising a contactor 276 which is movable between contacts 278 and280. The contact 278 is grounded, while the contact 280 is connected tothe positive power supply lead 236. The contactor 276 is connected tothe cathode of the SCR 234 through a resistor 282, across which anotherresistor 284 and an inductance coil 286 are connected in series. A diode288 is connected across the resistor 284. Bypass capacitors 290, 292,and 294 are connected between contactor 276 and ground.

For pulsed operation, the contactor 276 engages the grounded contact278. The diode 288 is non-conductive and the network comprising theresistor 282, the resistor 284 and the coil 286 act as a load for theSCR 234. When continuous operation is desired, the contactor 276 ismoved against the contact 280. The diode 288 and the coil 286 provide alow impedance bypassing circuit around the SCR 234.

The SCR pulsing system makes it possible to achieve very high sparkrepetition rates such as 5,000 Hz. or even higher. Only a low level ofpower is required for the triggering of the SCR 234. Thus, the pulsingcan readily be computer controlled. Moreover, the pulsing can besynchronized with any other desired signal.

FIGS. 5a and 5b are approximate equivalent circuit diagrams of theresonant transmission line 42 and the associated components. Eachcircuit diagram comprises a resistance 298 which represents the internalimpedance of the radio frequency source 64. The combination of the sparkgap 22 and the quarter-wave resonant line 42 produces an automaticswitching operation to change the impedance seen by the radio frequencysource 64. In the diagram of FIG. 5a the switching action of the sparkgap is represented by an equivalent switch 300 connected to the outputend of the quarter-wave resonant line 42. In FIG. 5b, an equivalentswitch 302 represents the automatic switching action of the spark gap,in combination with the quarter-wave line 42. Initially, before thespark is ignited, the spark gap 22 presents an open circuit, so that itsimpedance is virtually infinite. This condition is represented by theleft-hand position of the switch 300, in which the open spark gap 22 isconnected to the output end of the quarter-wave resonant line 42.

Due to the action of the resonant line 42, the impedance at the inputend of the line is very low, represented by a low value equivalentresistor 304, which is connected to the line 42 by the switch 302, inits left-hand position. The value of this equivalent resistor 304 may beonly a small fraction of 1 ohm, for example.

The quarter-wave resonance exists in the line 42 between the currentinjection point and the spark gap 22. This is the output portion of theline. The input portion, between the radio frequency generator 64 andthe current injector point, is tuned to a parallel resonant condition bythe variable capacitor 76. The reactance of the input portion of theline is represented by an inductance 306 in FIG. a. Due to the parallelresonant condition, the radio frequency generator 64 is presented withhigh impedance, so that it develops a high voltage.

When the spark is ignited, the spark gap presents a low impedance,represented by an equivalent resistance 308 in FIG. 5a. The value ofthis resistance may be only a few ohms, for example. The equivalentswitch 300 is shifted to its right-hand position in which the lowresistance 308 is connected to the output end of the resonant line 42.

Due to this change in output impedance, the input of the quarter-waveresonant line 42 appears as a fairly high equivalent resistance 310,connected into the circuit in the right-hand position of the switch 302in FIG. 5b. When this high impedance is presented to the parallelresonant circuit, the input impedance is greatly reduced. The automaticswitching action effectively decouples the radio frequency source 64from the quarter-wave resonant line 42, so that the transfer of power tothe line is greatly reduced.

Prior to spark ignition, the impedance presented at the currentinjection lead 72 is very low, so that virtually any current source canbe connected to this point. When the spark is ignited, the currentsource dumps its current into the spark discharge.

The operation of the spark source is illustrated by the oscillograms ofFIGS. 8-16, representing the voltage across the spark gap for variousconditions. FIG. 8 shows an oscillogramv 320 of the 162 MHz. voltage asit builds up to ignite the spark gap. The point of ignition isapproximately at 322, after which the magnitude of the radio frequencyvoltage decreases rapidly to a low level, due to the low impedance ofthe ionized spark gap. Due to the extremely high frequency of the radiofrequency voltage, the spark discharge is initiated with a high degreeof precision. Thus,'the ignition of the spark gap can readily be timedto coincide with other functions, such as the dumping of the injectioncurrent into the spark discharge.

FIG. 9 illustrates an oscillogram 324 representing a fully oscillatorydischarge current across the spark gap. This wave form is produced bydischarging a capacitor through the spark gap, with a low dampingfactor.

FIG. 10 shows an oscillogram 326, corresponding to the initial portionof the oscillogram 324, but with a much faster sweep, so as to magnifythe oscillogram. It will be seen that the oscillogram 326 revealsstepped transients 328.

FIG. 11 illustrates an oscillogram 330 representing a unidirectionalpulse of current, dumped into the spark discharge. Such a pulse canreadily be produced by discharging a capacitor into the spark gap, withsufficient resistive damping to make the discharge unidirectional. Inthe situation represented by FIG. 11, a length of coaxial cable wasconnected across the current injection input, to produce reflections,represented by the successive saw-tooth peaks 332 of the oscillogram330.

FIG. 12 illustrates an oscillogram 334 representing the dumping of ahalf sinusoid of current across the spark gap after the ignition of thespark. Such a current can readily be developed by a pulsed rectifier,deriving its input from an alternating current source.

FIG. 13 shows an oscillogram 336 representing a capacitor dischargethrough a low impedance line having distributed parameters.

FIG. 14 shows an oscillogram 338, representing the combineddistributions of FIGS. 12 and 13. It will be evident that a wide varietyof discharge wave forms can be produced by combining various discharges.

Another wave form of this type is represented by the oscillogram 340 ofFIG. 15. This is a unidirectional wave form.

A reversing pulse wave form is represented by the oscillogram 342 ofFIG. 16.

FIG. 17 shows an oscillogram 344 corresponding to the initial portion ofthe oscillogram 342 but with a much faster sweep, to producemagnification. The oscillogram 344 reveals stepped transientsrepresenting the precise ignition of the spark by the radio frequencyvoltage.

In summary, the present invention utilizes a radio frequency voltage,fed through a resonant transmission line, to ignite or form the spark.The line has an output section, tuned as a quarter-wavelength resonantline, and an input section, tuned to parallel resonance. Thequarter-wavelength resonant line section develops the high voltagenecessary to break down the spark gap.

Sustaining current is injected into the system at the nodal pointbetween the parallel resonant section and the quarterwavelengthresonant-section. This is a point of minimum radio frequency voltage sothat very little reactive isolation is necessary between the radiofrequency system and the injection current source.

Injection current pulses having a wide variety of waveforms can easilybe delivered to the formed spark by way of the injection point. Thesystem affords low impedance to. the delivery of such current pulses.Thus, a high current can be delivered at a low voltage. The currentinjection circuit can utilize low voltage components.

The ignition or formation of the spark by the high radio frequencyvoltage makes it possible to ignite the spark with a high degree ofprecision. Thus, thespark can be precisely synchronized with otherapparatus, or a wide variety of signalsv and events.

The radio frequency voltage can be pulsed in response to signals at alow power level. Thus, computer control of the pulsing is readilypossible. High spark repetition rates can readily be achieved. Moreover,the repetition rate can be varied continuously over a wide range, simplyby'changing the frequency of the triggering signal.

The coaxial resonant line provides complete shielding around the sparkgap, so that the spark discharge is not subject to outside radiofrequency interference. Moreover, the radiation ofradio frequency energyis minimized, so as to prevent interference with external radioequipment.

Due to the utilization of the coaxial resonant line the spark gap isaxially disposed in a cylindrical enclosure. It has been found that thisconfiguration results in stabilization of the spark so that it standsstill on the spark gap electrodes, particularly at high repetitionrates. It is believed that this stabilization of the spark is due toacoustical resonance in the cylindrical chamber around the spark gap.The shock waves from the spark are reflected within the cylindricalenclosure so as to stabilize the spark.

The stabilization of the spark makes it possible to achieve opticalresonance within the cylindrical enclosure, so that laser beams can beproduced. Such optical resonance requires the provision of a cylindricalmirror or reflecting surface within the enclosure.

The primary utility of the spark system is to provide for the emissionof light from the spark discharge, for use in optical emissionspectroscopy. The materials to be analyzed can be placed on the sparkgap electrodes, or introduced as gases into the spark discharge.

The spark system can also be used as a vapor or ion source for massspectroscopy. For such use, the vapors or ions produced by the sparkdischarge are introduced into a mass spectrometer.

Due to the stabilization of the spark, the system can be used for thecontrolled erosion of materials placed on the spark gap electrodes. Thesystem is also useful as a source of shock waves, which are produced bythe spark discharge.

Various other advantages, modifications and equivalents will be evidentto those skilled in the art.

The values of electrical components are not critical and can be assignedby those skilled in the art. However, it may be helpful to list suitablevalues of various components, as follows:

RESISTORS Ohms 210 lSK 212 114 240 120K 252 56K 254 56K 270 270 282 1KCAPACITORS 68 3x100 picofarads (pf.) 76 0.140 r. 122 I pf. 138 0.005microfarad (mf) 140 0.005 mi. 150 -40 pf. 168 3-20 pf. 160 180 pf. 162330 pf. 164 330 r. 176 1,500 pf. 178 180 pf. 180 330 pf. 182 330 pf. 190180 pf. 192 330 pf. 194 330 pf, 214 330 pf. 226 2.3l4.2 pf. 230 l.8-5.lpf. per section 242 330 pf. 244 0.0] mf. 246 330 pf.

248 0.0l mf.

256 0.01 mf. 25a 56 pf. :90 0.01 ml 292 0.01 mf. 294 330 mi.

We claim:

1. A spark source, comprising a resonant line having a pair ofconductors,

a spark gap connected across said conductors at one end of said line,

a radio frequency power source connected across said conductors at theopposite end of said line,

means for connecting a discharge current source across said conductorsat an intermediate injection point along said line,

and means for producing a radio frequency node at said injection pointto obviate interaction between said radio frequency power source andsaid discharge current source.

2. A spark source according to claim 1,

in which said line is coaxial,

one of said conductors being generally in the form of a cylinder whilethe other conductor is disposed axially within said cylinder.

3. A spark source according to claim 1,

in which said line has an electrical length of approximately one-fourthwave length between said spark gap and said injection point.

4. A spark source according to claim 3,

in which said line is coaxial,

one of said conductors being in the form of a cylinder while the otherconductor is axially disposed therein.

5. A spark source according to claim 1, including a tuning deviceconnected to said line.

6. A spark source according to claim 5,

in which said tuning device comprises a variable capacitor connectedbetween said conductors of said line.

7. A spark source according to claim 6,

in which said variable capacitor is connected to said line adjacent saidradio frequency power source.

8. A spark source according to claim 7,

in which said conductors of said line are substantially coaxi- 9. Aspark source according to claim 1,

in which said conductors of said line are coaxial,

one of said conductors being generally in the form of a cylinder whilethe other conductor is axially disposed therein,

said spark gap comprising a first axial electrode connected to the endof said axial conductor,

and a second electrode disposed axially in said cylinder and spacedaxially away from said first electrode,

said second electrode being connected to said cylinder.

10. A spark source according to claim 9,

in which said cylinder fonns a chamber around said spark gap and isacoustically resonant for stabilizing the spark discharge across saidgap.

1 l. A spark source according to claim 10,

including an end wall closing said cylinder at the end thereof adjacentsaid spark gap.

12. A spark source according to claim 11,

in which said second electrode is mounted upon said end wall andprojects therefrom into said cylinder,

13. A spark source according to claim 12,

including means for adjusting said second electrode toward and away fromsaid first electrode.

' 14. A spark source according to claim 1,

including means for adjusting the length of said spark gap.

15. A spark source according to claim 9,

in which one of said electrodes is formed with a longitudinal passagefor injecting materials into the spark gap.

16. A spark source according to claim 9,

in which said first electrode comprises an axial passage for injectingmaterial into the spark gap 17 A spark source according to claim 16,

including a pointed electrode pin mounted axially in said passage,

said pin being smaller than said passage to provide for the flow ofmaterials through said passage and along said pin.

18. A spark source,

comprising a hollow member having an internal cylindrical wall forming acylindrical chamber in said member,

a first spark gap electrode disposed axially within said chamber,

a second spark gap electrode disposed axially in said chamber and spacedaxially from said first electrode to form an axial spark gaptherebetween,

a high voltage source connected to said first and second spark gapelectrodes for producing sparks between said electrodes and across saidgap,

each spark producing an acoustical shock wave which travels outwardly tosaid cylindrical wall and is reflected inwardly by said wall,

and pulsing means for pulsing said high voltage source to producerepetitive sparks between said electrodes at a repetition rate relatedto the internal size of said chamber such as to produce acousticalresonance in said chamber in response to the repetitive shock waves,

the repetitive sparks being stabilized by said acoustical resonance.

19. A spark source according to claim 18,

including an end wall closing one end of said cylindrical chamber,

said second electrode being mounted upon said end wall.

20. A spark source according to claim 18,

including means for axially adjusting one of said electrodes to changethe length of said spark gap between said electrodes.

21. A spark source according to claim 18,

including an end wall across one end of said cylindrical chamber,

said second electrode being mounted upon said end wall,

and another end wall extending across said cylindrical chamber andspaced axially from said end wall,

said spark gap between said electrodes being disposed in said chamberand between said end walls.

22. A spark source according to claim 18,

said cylindrical wall being conductive and being connected to saidsecond electrode,

said high voltage source being connected between said first electrodeand said cylindrical wall,

the dimensions of said cylindrical wall and said first electrode beingsuch as to produce electromagnetic resonance in said cylindricalchamber.

1. A spark source, comprising a resonant line having a pair ofconductors, a spark gap connected across said conductors at one end ofsaid line, a radio frequency power source connected across saidconductors at the opposite end of said line, means for connecting adischarge current source across said conductors at an intermediateinjection point along said line, and means for producing a radiofrequency node at said injection point to obviate interaction betweensaid radio frequency power source and said discharge current source. 2.A spark source according to claim 1, in which said line is coaxial, oneof said conductors being generally in the form of a cylinder while theother conductor is disposed axially within said cylinder.
 3. A sparksource according to claim 1, in which said line has an electrical lengthof approximately one-fourth wave length between said spark gap and saidinjection point.
 4. A spark source according to claim 3, in which saidline is coaxial, one of said conductors being in the form of a cylinderwhile the other conductor is axially disposed therein.
 5. A spark sourceaccording to claim 1, including a tuning device connected to said line.6. A spark source according to claim 5, in which said tuning devicecomprises a variable capacitor connected between said conductors of saidline.
 7. A spark source according to claim 6, in which said variablecapacitor is connected to said line adjacent said radio frequency powersource.
 8. A spark source according to claim 7, in which said conductorsof said line are substantially coaxial.
 9. A spark source according toclaim 1, in which said conductors of said line are coaxial, one of saidconductors being generally in the form of a cylinder while the otherconductor is axially disposed therein, said spark gap comprising a firstaxial electrode connected to the end of said axial conductor, and asecond electrode disposed axially in said cylinder and spaced axiallyaway from said first electrode, said second electrode being connected tosaid cylinder.
 10. A spark source according to claim 9, in which saidcylinder forms a chamber around said spark gap and is acousticallyresonant for stabilizing the spark discharge across said gap.
 11. Aspark source according to claim 10, including an end wall closing saidcylinder at the end thereof adjacent said spark gap.
 12. A spark sourceaccording to claim 11, in which said second electrode is mounted uponsaid end wall and projects therefrom into said cylinder,
 13. A sparksource according to claim 12, including means for adjusting said secondelectrode toward and away from said first electrode.
 14. A spark sourceaccording to claim 1, including means for adjusting the length of saidspark gap.
 15. A spark source according to claim 9, in which one of saidelectrodes is formed with a longitudinal passage For injecting materialsinto the spark gap.
 16. A spark source according to claim 9, in whichsaid first electrode comprises an axial passage for injecting materialinto the spark gap
 17. A spark source according to claim 16, including apointed electrode pin mounted axially in said passage, said pin beingsmaller than said passage to provide for the flow of materials throughsaid passage and along said pin.
 18. A spark source, comprising a hollowmember having an internal cylindrical wall forming a cylindrical chamberin said member, a first spark gap electrode disposed axially within saidchamber, a second spark gap electrode disposed axially in said chamberand spaced axially from said first electrode to form an axial spark gaptherebetween, a high voltage source connected to said first and secondspark gap electrodes for producing sparks between said electrodes andacross said gap, each spark producing an acoustical shock wave whichtravels outwardly to said cylindrical wall and is reflected inwardly bysaid wall, and pulsing means for pulsing said high voltage source toproduce repetitive sparks between said electrodes at a repetition raterelated to the internal size of said chamber such as to produceacoustical resonance in said chamber in response to the repetitive shockwaves, the repetitive sparks being stabilized by said acousticalresonance.
 19. A spark source according to claim 18, including an endwall closing one end of said cylindrical chamber, said second electrodebeing mounted upon said end wall.
 20. A spark source according to claim18, including means for axially adjusting one of said electrodes tochange the length of said spark gap between said electrodes.
 21. A sparksource according to claim 18, including an end wall across one end ofsaid cylindrical chamber, said second electrode being mounted upon saidend wall, and another end wall extending across said cylindrical chamberand spaced axially from said end wall, said spark gap between saidelectrodes being disposed in said chamber and between said end walls.22. A spark source according to claim 18, in which said high voltagesource includes means for producing a high voltage at a high frequency.23. A spark source according to claim 18, in which said cylindrical wallis conductive and is connected to one of said electrodes.
 24. A sparksource according to claim 18, in which said high voltage sourcecomprises means for producing a high voltage at a high frequency, saidcylindrical wall being conductive and being connected to said secondelectrode, said high voltage source being connected between said firstelectrode and said cylindrical wall, the dimensions of said cylindricalwall and said first electrode being such as to produce electromagneticresonance in said cylindrical chamber.